Low frequency flextensional acoustic source for underwater use

ABSTRACT

Acoustic source, especially for use in seismic studies at sea, with sound emitting surfaces adapted to be put into sound emitting movements toward and away from each other by use of a drive unit in a known manner, in which the sound emitting surfaces comprise two curved plate members essentially symmetrically positioned in relation to the longitudinal axis of the source, forming a convex cross section with their respective upper and lower edges converging towards each other, and the respective upper and lower edges of the sound emitting surfaces are flexibly connected to two corresponding side elements positioned between them, and that the source comprises a rigid frame for supporting the drive unit, comprising at least two rigid slide rods stretching through the side elements, and that rigid covers are mounted on the slide rods, covering at least the whole side elements, thus forming rigid, covering surfaces with a constant distance between the covers.

This invention relates in general to acoustic sources for the generationof soundwaves in water, especially soundwaves with low frequencies, e.g.for use in seismic studies at sea. Such sources, being used ingenerating soundwaves in water, may be described in different ways, suchas seismic emitters, flextensional sources or acoustic vibrators, suchas sonars.

The acoustic source according to the invention may preferably be used inrelation to such sound- or toneemitters, since the soundwaves emittedinto the water may be reflected from the sea bottom and geologicalformations beneath it, with succeeding monitoring and detection usinghydrophones or geophones of different types.

The invention is especially aimed at a certain embodiment of soundemitting membranes or shells in acoustic sources, and the presentembodiment may be considered to be related to certain types offlextensional emitters.

Flextensional emitters are described in many publications, one examplebeing L. H. Royster:"The flextensional concept. A new approach to thedesign of underwater acoustic transducers", Appl. Acoust. 3 (1970), pp117-226. Other examples are G. Bromfield: "Class IV flextensionaltransducers", Proceedings Second International Workshop on PowerTransducers, Toulon, (France) June 1990, and G. Brigham and B. Glass:"Present status in flextensional transducer technology", J. Acoust. Soc.A., 68(1980), pp. 1046-1052.

Most flextensional shells are based on the principle of changes involume in a vibrating, elliptic shell. When the long axis of an ellipseis set into vibration the length of the short axis will also vibrate,but with a much larger amplitude. This makes the ellipse shape aneffective generator of acoustic energy at low frequencies. Generally thebasic resonance frequency of the elliptic shell is so low that thelength of the shell is small when compared to the wavelength in water.

A limiting factor for this kind of shells are the mechanical tensionsforced upon the shell when large vibrational amplitudes are to beobtained. Normally aluminum or glass-fibre armed plastic is used asshell material. The use of such shells are also limited as a result oftheir complex shape, which lead to complicated manufacturing methods.

A special example of a complicated shape, being visually andfunctionally different from the abovementioned elliptic, round shells(egg-shaped), are hyperbolic, round shells. The advantage with thisshape is that the short axis will vibrate in face with the long axis. Anembodiment based on this principle is described in D. F. Jones and C. G.Reithmeier: "Low frequency barrel-stave projectors" (Proceedings UDT 93,Nice, France, Microwave Exh. & Pub. Ltd. 1993, pp 251-253).

This last embodiment comprises in short two octagonal end plates joinedtogether with eight rod-shaped elements having a concave curvature andbeing positioned in an essentially circular or polygonal arrangementsurrounding an axis of symmetry. A major problem with this design is themany slits between the rod elements, and the question of sealing andpossible contact between these elements.

In Norwegian patent application no. 94.1708 (corresponding toInternational patent application no. PCT/NO95/00071) an acoustic sourceis described with sound emitting surfaces having a generally concaveshape and being adapted to be set into vibrational movement using adrive assembly affecting the sound emitting surfaces through at leastone pressure element and two platelike end parts being connected to thesound emitting surfaces. The novelty of this invention is primarilyrelated to the fact that the sound emitting surfaces are formed by twocurved membranelike plate members having width and height dimensions ofcomparable sizes and being mounted back-to-back, that the plate-like endpieces have essentially rectangular shapes with a first pair of oppositeside edges connected to the end parts of the plate members, and thatanother pair of side edges on each end piece, as well as the side edgesof the plate members are connected to two side pieces which close theacoustic source on each side, preferably in a sealing way.

An object of this invention is to provide an improved acoustic source,which through a simple and suitable embodiment provides a dependable andreliable source unit, at the same time having a high acousticefficiency.

On the basis of the mentioned prior art this invention is thus based onan acoustic source primarily for use in seismic studies at sea, withsound emitting surfaces adapted to be put into sound emitting movementto and from each other using a drive unit in an, in itself, known way.

The new and inventive features according to this invention is that thesound emitting surfaces are formed by two curved plate membersessentially symmetrically positioned in relation to the axis of thesource forming a convex cross section with the upper and the lower edgesconverging towards each other,

that the upper and lower edges of the sound emitting surfaces areflexibly connected to two corresponding side elements positioned betweenthem,

and that the source comprises a rigid frame, preferably for the supportof the drive assembly, comprising at least two rigid sliding rodsstretching between the side elements, and that covers are mounted on theslide rods being adapted to cover essentially all of the side elementsand possibly parts of the sound emitting surfaces in order to formrigid, covering surfaces with constant distance between them.

The invention is in the following described with reference to theaccompanying drawings, in which:

FIG. 1 shows a section of one embodiment of an acoustic source accordingto the invention.

FIG. 2 shows an alternative embodiment according to the invention.

FIG. 1 shows an embodiment of the invention with an essentially circularcross section comprising sound emitting surfaces 2 being in their upper4 and lower 5 ends flexibly connected by rubber members 17 or otherflexible material to side elements 6. The side elements 6 are coveredwith covers 10. When the side elements 6 are pulled towards and awayfrom each other using the drive assembly 3,12,13 the sound emittingsurfaces are respectively pushed away from and towards each other, andgenerates thus a pressure wave which may propagate through water.Normally the side elements 6 in an acoustic source with the shape shownin FIG. 1 would generate acoustic waves with a phase being oppositecompared to the acoustic waves generated from the sound emittingsurfaces 2. In order to prevent this, and at the same time maintain thecompact shape, the acoustic source is provided with covers 10 coveringthe side elements 6 and keeping a constant distance from each otherusing the slide rods 8 and the frame 7. The slide rods 8 have a sliding,and possibly sealed, connection to the side elements, using slidebearings 19.

The drive unit 3,12,13 is mounted inside a frame 7. The frame 7 is heldin position inside the source using the slide rods 8 drawn vertically inthe drawings. Preferably horizontal slide rods 11 are also used to lockthe position of the frame 7 in the vertical direction. These may forexample be mounted using slide bearings 21 to the outer drive parts 3.If other types of drive units are used these second slide rods 11 may bemounted in suitable suspension points or in relation to, but usually notthrough, the sound emitting surfaces.

The covers 10 are preferably tightly connected to the source withsealing strips 9 or similar devices, so that the movements of the sideelements do not propagate through the surrounding water. Thus the signalfrom the sound emitting surfaces 2 will not be distorted by signals fromthe side elements 6. This gives an advantage in that the dimensions ofthe sound emitting surfaces may be chosen independently of size of theside elements 6. If the room between the covers and the side elementscontains air the friction because of the movements of the side elementswill not influence the output gain of the source in any great degree. Inthis case the slide bearings 19 around the slide rods 8 do not need tobe tight.

Corresponding parts may, however, also be used in combination with otherkinds of sources, such as previously known elliptic sources. In FIG. 2 asource is shown with an essentially elliptic cross section, where theside elements 6 are made small by using hinger 15 connecting the upperand lower edges 4,5, respectively, of the sound emitting surfaces. Asimilar solution in which two rows of hinges 15, each connecting theside elements 6 with the edges 4,5 of one of the sound emitting surfaces2, may of course also be contemplated.

The acoustic sources shown in the drawings are equipped withelectrodynamic drive units. These are described in detail in Norwegianpatent application no. 95.2605, (corresponding to International patentapplication no. PCT/NO96/00131). In short the drive units comprise pairsof electromagnetic and magnetic elements 3,12 being given a relativemovement in relation to each other by applying a varying electriccurrent. One of these parts 12 are mounted firmly in relation to a frame7, and the other 3 is mounted on a transmission device 13. A movement ofthe second part 3 gives the transmission device 13 a movement whichpropagates to the side elements 6, which in turn moves the soundemitting surfaces 2. Other types of known drive units, for example basedon electric rotation motors like the ones described in Norwegian patentapplication no. 95.1949 (corresponding to International patentapplication no. PCT/NO96/00119) or Norwegian patent no. 176.457(corresponding to International patent application no. PCT/NO94/00057),may of course also be used.

The dimensions of the acoustic source in the longitudinal direction (notshown in the figures) will vary, depending on use, and is not importantto the invention. The number of slide rods 8,11, and drive units 3,12,13will depend on the length of the source. In the drawings the slide rodsare shown in a position centrally on the covers. Other solutions, suchas slide rods positioned in pairs side by side along the length of thesource, will also be possible.

I claim:
 1. Acoustic source having a longitudinal axis, especially foruse in seismic studies at sea, with sound emitting surfaces adapted tobe put into sound emitting movements towards and away from each other byuse of a drive unit in which the sound emitting surfaces comprise twocurved plate members essentially symmetrically positioned in relation tothe longitudinal axis of the source, forming a convex cross section withtheir respective upper and lower edges converging towards each other,and the respective upper and lower edges of the sound emitting surfacesare flexibly connected to two corresponding side elements positionedbetween the upper and lower edges of the sound emittingsurfaces,characterized in that the source comprises a rigid frame forsupporting the drive unit comprising at least two rigid slide rodsstretching through the side elements and permitting relative axialmovement of the slide rods with respect to the side elements, and thatrigid covers having outer edges are mounted on the slide rods, coveringat least the whole side elements, thus forming rigid, covering surfaceswith a constant distance between the covers.
 2. Acoustic sourceaccording to claim 1,characterized in that the edges of the covers aresealed to provide a watertight contact between the covers and the restof the source.
 3. Acoustic source according to claim 1,characterized inthat the side elements are connected to one or more hinges suitable forfastening each of the upper and lower edges, respectively, of the soundemitting surfaces to each other or to the side elements.
 4. Acousticsource according to claim 1,characterized in that the coupling betweenthe side elements and the sound emitting surfaces comprises rubber. 5.Acoustic source according to claim 1,characterized in that the sideelements comprises side plates.
 6. Acoustic source according to claim1,characterized in that the drive unit is an electromagnetic drive unit.7. Acoustic source according to claim 1, characterized in that the sliderods are fastened to the side elements with slide bearings.
 8. Acousticsource according to claim 1, characterized in that the frame alsocomprises slide bearings associated with the drive unit and at least twoadditional second slide rods, mounted essentially perpendicular to thelongitudinal axis of the source and mounted in the slide bearingsassociated with the drive unit.